1. Field of the Invention
The present invention relates to a framing identification and timing signal extraction apparatus for a very small aperture terminal(VSAT) system and a method thereof, and particularly to an improved framing identification and timing signal extraction apparatus for a very small aperture terminal system and a method thereof which are capable of directly extracting three kinds of framing identification and timing (FIT) signals from a user terminal for a synchronization between a central unit and a terminal unit, for thus using the FIT signals for a synchronization of the system wherein the very aperture terminal system for communicating a predetermined information between a user terminal and a main frame of a central unit which are connected to a plurality of terminal units by using a satellite.
2. Description of the Conventional Art
FIG. 1 is a block diagram illustrating a very small aperture terminal system.
As shown therein, a central unit 1 is connected for controlling the operation of all networks by using a satellite. A plurality of terminal units 2 are connected for performing communication operation through the central unit 1. A network management system 3 is connected for controlling an information communication between the central unit 1 and the terminal systems 2. A main frame 4 is connected with the central unit 1 for storing information to be transmitted through network lines.
Here, in the very small aperture terminal system, a data is transmitted by a carry signal or a plurality of continuous carry signals from the central unit 1 to the terminal units 2. This data is conventionally called an outbound data.
In addition, the terminal units 2 share one carry signal and transmit a predetermined data to the central unit 1. Here, this data is called an inbound data.
Here, various channel access protocols are used so that the plurality of the terminal units 2 share one input type carry. The most commonly used channel access protocols are a random aloha and a slotted aloha.
Meanwhile, the data packet transmitted to the terminal units 2 from the central unit 1 has two different types of information.
The first type of information is user data communicated between the central unit 1 and the terminal units 2. The carrier of one outbound has a plurality of user data.
The second type of information is a synchronous information signal for a signal transmission apparatus.
This information signal is a bit signal such as framing bits, identification bits, and timing bits and is generated by a modulator (not shown) of the central unit 1. This bit signal is extracted from the receiver (not shown) of the terminal unit 2. The thusly extracted bit signal is used for a synchronization for transmitting a burst data to the central unit 2.
FIG. 2 is a view illustrating the construction of a bit signal of a framing identification and timing (FIT) signal according to the present invention. As shown therein, framing bits F8 through F0 among bit signals as shown in FIG. 2 are formed in a 9-bit form of "011111111" which was the fitted value upon transmission at the central unit 1. These framing bits F8 through F0 are used for separating the FIT information and the user data from the data packet inputted to the terminal unit 2.
Identification bits I8 through I0 are used for searching outbound carrier used in the specific terminal unit among a plurality of outbound carriers.
The timing bits T8 through T0 denote one slot among 0-99 slots. The timing bits are used for synchronizing slots to entire terminal units.
Namely, as shown in FIG. 2, each FIT bit has 9 bits. There are 25 bits between each FIT bit. 25 bits comprise 1 FIT bit and 24-bit user data. There are 75 bits between F0 bit and F1 bit. Accordingly, there are 695 bits between F0 bit and T9 bit. The clock period between F0 and I0 is 0.38 ms. (2.666 KHz). The clock period between F0 and F1 is 1.13 ms. (0.888 KHz). Therefore, the entire synchronizing clock time is 10.17 ms. The FIT signal and user data are loaded in carrier frequency.
However, since the conventional very small aperture terminal system is not directed to directly extracting the FIT signal from a user terminal 5, there may be many limitations in using the user terminal 5.